1. Field of the Invention
The present invention relates generally to global navigation satellite systems (GNSS), and in particular to removing biases in dual frequency GNSS receivers.
2. Description of the Related Art
The Global Positioning System (GPS) was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted as L1 and L2 respectively. These signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error. Different levels of accuracies can be achieved depending on the techniques deployed. For example, processing carrier phase observations both from a mobile/remote receiver and from one or more fixed-position reference stations is often referred to as Real-Time-Kinematic or RTK, and can produce sub-centimeter accuracy.
To gain a better understanding of the accuracy levels achievable by using the GPS system, it is necessary understand the two types of signals available from the GPS satellites. The first type of signal includes both the Coarse Acquisition (C/A) code, which modulates the L1 radio signal and the precision (P) code, which modulates both the L1 and L2 radio signals. These are pseudorandom digital codes that provide a known pattern that can be compared to the receiver's version of that pattern. By measuring the time-shift required to align the pseudorandom digital codes, the GPS receiver is able to compute an (DN 4165)
unambiguous pseudo range to the satellite. Both the C/A and P codes have a relatively long “wavelength,” of about 300 meters (1 microsecond) and 30 meters ( 1/10 microsecond), respectively. Consequently, use of the C/A code and the P code yield position data only at a relatively coarse level of resolution.
The second type of signal utilized for position determination is the carrier signal. The term “carrier”, as used herein, refers to the dominant spectral component which remains in the radio signal after the spectral content caused by the modulated pseudorandom digital codes (C/A and P) is removed. The L1 and L2 carrier signals have wavelengths of about 19 and 24 centimeters, respectively. The GPS receiver is able to “track” these carrier signals, and in doing so, make measurements of the carrier phase to a small fraction of a complete wavelength, permitting range measurement to an accuracy of less than a centimeter.
Satellite-based augmentation systems (SBASs), such as the FAA's Wide Area Augmentation System (WAAS) and the European Geostationary Navigation Overlay Service (EGNOS), broadcast correction components for global navigation satellite system (GNSS, including the Global Positioning System (GPS)) positioning that include ionosphere correction maps, fast clock correctors, slow clock correctors and orbit correctors. A single-frequency GPS receiver receives these components over an SBAS broadcast signal, and using a troposphere model, differentially corrects its measured pseudo ranges, ultimately improving receiver positioning accuracy.
One of the problems with existing SBAS systems is that they are designed for use with single-frequency receivers, with which the ability of the SBAS system to correct ionosphere errors is one of the main limitations to achieving higher accuracy. SBAS systems model the ionosphere as a thin shell and fit ionosphere delay readings obtained with a network of dual-frequency GPS receivers to this shell. SBAS satellites then broadcast a vertical delay map of the shell to SBAS-enabled GPS receivers so that the single-frequency GPS receiver can correct ionosphere errors in its measured pseudo ranges. In the receiver, vertical delays are interpolated from the map and scaled according to an obliquity factor. However, the SBAS approach to correcting ionosphere errors is a first order approach and can have errors exceeding one half meter during normal operation and even tens of meters during high solar activity. This is particularly true as ionosphere gradients become large and the assumption of a thin shell breaks down.
Whitehead U.S. Pat. No. 6,397,147 discloses real-time, single-receiver relative GPS positioning using a technique where differential correction terms are computed at particular locations and instants of time, adjusted for atmospheric delays and then applied at later instants of time. The later GPS-defined positions are thus determined accurately relative to the earlier positions because the ionosphere errors are canceled out. This patent is assigned to a common assignee herewith and is incorporated herein by reference. Such relative positioning accuracy is often sufficient for applications not requiring absolute positioning accuracy, such as agricultural and machine control operations where the primary concern is positioning the equipment relative to its initial location or starting point. For example, agricultural equipment is often guided over fields in parallel swaths which are preferably located accurately relative to each other, but need not be precisely positioned in absolute GPS or other earth-based coordinates.
SBAS systems are designed to correct only L1(C/A) pseudo ranges including the problematic ionosphere delay component, but a dual frequency receiver can circumvent the need for ionosphere corrections by using L2(P) in combination with either L1(P) or L1(C/A) to form the ionosphere-free ranges. A bias, known as the inter-frequency bias, exists between L1(P) and L2(P). This bias is different for each GPS space vehicle and takes on a value ranging from a fraction of a meter to a couple of meters. The GPS satellites broadcast this bias in a term known as Tau-Group-Delay (τGD), but due to word size, this broadcast has a limited resolution of 0.14 meters.
There is another bias, on the order of a few decimeters, between L1(C/A) and L1(P) that is also satellite dependant, but is not broadcast over the GPS navigation message (today). This bias is the inter-signal group delay code bias, referred to in the modernized GPS ICDs as ISCL1C/A. Various organizations, particularly the Center for Orbit Determination in Europe (CODE) predict and maintain estimates of ISCL1C/A using a global network of monitoring Dual-Frequency GPS receivers.
A good discussion of these biases can be found in “Dual-Frequency GPS Precise Point Positioning with WADGPS Correctors” by Hyunho Rho and Richard Langely, Navigation Journal of The Institute of Navigation, Summer 2007, No. 2, Vol. 54, pp. 139-151.